This invention relates to an LED (light emitting diode) array head which is used for an LED printer or like apparatus employing an electrophotographic technique and also to a process of producing such an LED array head.
General construction of a conventional LED printer in which an LED array head is used will first be described with reference to a schematic view of FIG. 6.
The LED printer shown in FIG. 6 includes a photosensitive drum 1 having a surface layer made of a photoconductive material. The photosensitive drum 1 is driven to rotate in a direction indicated by an arrow mark in FIG. 6. During such rotation, the drum 1 is charged uniformly by a charger 2 and is then exposed to light in accordance with information of a picture image to be recorded by an LED array head 3 in order to form an electrostatic latent image on the drum 1. The latent image is then developed by a developer 4 into a toner image which is subsequently transferred onto transfer paper 6 by a transfer device 5. The transfer paper 6 to which the toner image has been transferred is then advanced to a fixing device 7 at which the toner image is fixed to the paper 6. Meanwhile, the photosensitive drum 1 from which the toner image has been transferred is then cleaned by a cleaner 8 in order to allow subsequent re-use thereof.
Such LED array heads as the head 3 conventionally include an LED element array including a large number of LED chips arranged in a direction of printing columns as described hereinbelow, and a self-focusing type rod lens array for condensing light emitted from the LED elements of the element array to focus on a photosensitive drum.
An exemplary one of such conventional LED array heads will now be described with reference to FIG. 7. The LED array head 3 shown includes a large number of LED chips 32 die-bonded to a ceramics substrate 31, bonding wires not shown individually connected to the LED chips 32 and wire-bonded to respective light emitting portions of the LED chips 32 for energizing the LED light emitting portions, a cover glass 36 for protecting conductor patterns on the ceramics substrate 31 on which the LED chips 32 are mounted, a heat radiating plate 33 of aluminum or a like material on which the substrate 31 and the cover glass 36 are mounted, and a mounting plate 36 supported on the heat radiating plate 33 for mounting a self-focusing type rod lens array 34 thereon.
In such an LED array head 3 as described above, when the LED chips 32 are to be mounted in position on the ceramics substrate 31, the positioning tolerance for the linearity and so on must necessarily be within .+-.10 microns or so. Such positioning requires an optical technique involving a TV camera or a like device because the width of a pad for die bonding is 1.2 to 1.5 mm or so and is thus significantly great while the width of the LED chips 32 is 0.7 to 1 mm or so. Use of such an optical technique results in low efficiency in die bonding of LED chips and involves use of an expensive mounting apparatus.
Meanwhile, wire bonding must be done once for each of light emitting portions of each LED (each LED normally includes up to 64 light emitting portions). For example, in an LED array head for the A4 size and letter size having an integration density of 300 DPI (about 12 dots/mm) for ordinary use, up to 2560 wire bonds are required. Accordingly, even where a high speed wire bonding machine is employed, a time from 40 minutes to one hour is required for such bonding. Besides, even a single error among such 2560 wire bonds will result in rejection of the entire LED array head. Thus, very careful attention is required for wire bonding, which deteriorates the efficiency in production and raises the production cost.
Besides, since the accuracy in alignment of LED chips of an LED array head and the linearity in a direction of height of light emitting portions of the LED chips require such a high accuracy that the tolerance be within .+-.100 microns in the overall width of the LED array head (the width being 216 mm in A4 size or letter size), distortion or warping of a ceramics substrate itself must be restricted severely. Accordingly, a ceramics substrate of a high accuracy is required and therefore severe inspection is required for total number of ceramics substrates, which makes the ceraics substrates expensive.
In addition, there is a further drawback that, since a self-focusing type rod lens array is carried on a heat radiating plate on which a ceramics substrate is mounted, errors in dimension of those components will accumulate so that a required accuracy may not be assured. In the existing circumstances, the positioning tolerance of a self-focusing type rod lens array is within .+-.0.1 mm over the overall length of the same. Accordingly, optical confirmation of the positioning accuracy in mounting of a self-focusing type rod lens array cannot be eliminated, and it is necessary to provide an LED array head with a mechanism for enabling positioning the same with a high degree of accuracy, which raises the production cost from the points of a number of man-hours for assembly, an installation for manufacture, a quantity of parts and so on.